Sodium oxalate is a by-product of the Bayer refinery process used for producing aluminium hydroxide and alumina from bauxite. In the Bayer process, sodium oxalate is removed from sodium aluminate liquor, following or during aluminium hydroxide precipitation, by feeding the sodium aluminate liquor or liquor enriched in sodium oxalate to a crystalliser containing seed crystals of sodium oxalate. These seed crystals induce precipitation of further sodium oxalate. Sodium oxalate is normally removed by thickening and/or filtering the sodium oxalate slurry. Sodium oxalate is an environmentally hazardous compound and must be disposed of carefully. Typically the filtered sodium oxalate is reduced to solid cake form and must be transported to another site for disposal by combustion or other means.
Another method currently employed for the disposal of sodium oxalate from the Bayer process involves chemical treatment of the oxalate with lime. However, one of the products of this chemical treatment is calcium oxalate which is equally difficult to dispose of in an environmentally acceptable manner.
Australian Patent Application No. 39465/89 in the name of ALCAN INTERNATIONAL LTD describes a method for the biodegradation of oxalate in solution which has been removed from a Bayer processing system. Micro-organisms for degrading oxalates are maintained in a bio-reactor. Preparatory to introducing the oxalate-containing composition to the bio-reactor the pH of the oxalate-containing solution is adjusted to be compatible with the micro-organisms in the medium. In the Alcan method the pH of the solution is adjusted to the neutral range by the addition of a suitable acid, such as sulphuric acid. In addition, it may also be necessary to adjust the concentration of Na.sup.+ ions in the solution to a level which can be tolerated by the micro-organism developed to digest the oxalates. The micro-organisms employed were isolated from the soil of the rhizospheres of the oxalate producing plants Dieffenbachia picta and Rheum rhaponticum (Rhubarb), and it was believed that the prominent micro-organism which is capable of degrading oxalates is a Pseudomonas or Pseudomonas-like organism.
A significant disadvantage of the Alcan method of biodegradation of oxalate is the step of pretreatment of the oxalate-containing solution to adjust the pH to the neutral range and to reduce the sodium concentration to a level compatible with the micro-organisms. This pretreatment is necessary because the micro-organisms employed require an aqueous solution with pH in the neutral range and sodium concentration below a level of approximately 8000 mg/L. A further disadvantage is the need for an extended period of pre-stressing the microbial culture by exposure to varying concentrations of oxalates to ensure efficient oxalate degradation in the harsh environment of the oxalate-containing solution.
The present invention was developed with a view to providing a more economic and environmentally acceptable method of oxalate disposal using an oxalate-degrading micro-organism as the biological agent.